Construction of low water level sensing device for hot water boiler

Abstract

A low liquid level sensor in a hot water heating system permitting testing operability thereof in situ without draining down the system. The sensor is located in a cavity that has an opening in the bottom thereof in fluid communication with the liquid in the vessel during normal operation of the system. The end of the opening is at a lower elevation than the sensor and liquid from the vessel is in the cavity during normal operation of the system. When the liquid drops to a predetermined low level the sensor activates the safety cut-off. Valved passed means are provided exteriorly of the vessel so that a gas can be injected into the cavity to pressurize the same and drive the liquid in the cavity through the opening into the vessel simulating a low water level condition. At the simulated low water level condition operability of the sensor can be determined. Means are provided to release the gas from the cavity. The sensor is mounted by an insulator that projects into the cavity and the gas release means is spaced from the end of the cavity so that an air pocket is maintained reducing the tendency of conductive deposits to build up on the insulator. A fail safe circuit is provided avoiding malfunctioning of the cut-off safety control should there be a large build up of conductive deposits on the insulator.

I claim: 
     
       1. In a pressurized heating system having water in a boiler and a low water level sensor and safety cut-off device associated with the boiler and operable to shut off the system when the water in the boiler drops below a predetermined level, said low water sensor device comprising: (a) a housing having a cavity;   (b) means mounting said housing on the vessel such that a substantial portion of said housing, having the cavity therein, is external of the vessel;   (c) a sensor in said cavity responsive to the liquid level therein and operably associated with the heating system to shut off the same when the liquid level in the boiler drops below a predetermined level;   (d) a passageway in said housing having an open inlet end immersed in the water during normal operation of the boiler, said passageway providing fluid flow communication between the interior of the vessel and said cavity, the inlet to such passage being at a lower vertical elevation than the level of the water in the cavity at which the sensor is operative to shut off the system in a low water condition;   (e) valved passage means in the portion of said housing disposed exteriorly of the vessel for injecting a gas into said cavity to pressurize the cavity above the normal pressure therein; and   (f) normally open valved passage means in the portion of the housing located exteriorly of the vessel and connected to some higher point in the system such as an accumulator providing means for releasing the gas from said cavity and equalizing pressure between the cavity and the boiler.   
     
     
       2. A system as defined in claim 1 wherein said gas release means in spaced a selected distance downwardly from an upper end of said cavity and wherein the inlet end of the passageway is located adjacent an opposite end of the cavity. 
     
     
       3. A system as defined in claim 2 wherein said sensor is an elongate electrode suspended from the housing at the upper end of the cavity. 
     
     
       4. A system as defined in claim 3 wherein said electrode is suspended from an insulator that projects into the cavity and wherein the gas release means is at a lower elevation than the insulator. 
     
     
       5. A system as defined in claim 4 wherein said housing is threaded into a threaded bung in the upper wall of the boiler. 
     
     
       6. A system as defined in claim 5 wherein said housing includes a lower portion that projects into the boiler beyond the upper wall thereof. 
     
     
       7. A low water sensing device for use in a pressure hot water heating system to operate a safety cut-off when water in the boiler drops below a predetermined level, said device comprising a housing having an elongate cavity therein open at one end and closed at the opposite end; an elongate electrode suspended from an insulator mounted in said housing at said opposite end of the cavity, the axial length of said electrode within said cavity being less than that of the cavity whereby the end of the electrode is offset upwardly from the open lower end of the cavity; a first valved air inlet passage means through said housing into said cavity adjacent said opposite end thereof; a second air release passage means through said housing spaced a predetermined distance from said opposite end of the cavity in a direction toward said one end; and means located between said second air release passage means and said one end of the cavity for sealingly mounting said housing in an opening in the upper wall of the boiler of the pressure hot water heating system with the open end thereof in fluid flow communication with the interior of the boiler. 
     
     
       8. A device as defined in claim 7 wherein said insulator projects into said cavity. 
     
     
       9. A device as defined in claim 7 wherein said housing comprises an upper, an intermediate and a lower portion and wherein said portions are detachably interconnected. 
     
     
       10. A device as defined in claim 9 wherein said boiler mounting means is located on said intermediate portion of said housing. 
     
     
       11. A device as defined in claim 10 wherein one end of the lower portion of said housing is clampingly disposed between said upper and intermediate portions of said housing. 
     
     
       12. A device as defined in claim 10 wherein each of said first and second air passage means are located in the upper portion of said housing. 
     
     
       13. A method of testing operability of the sensor of a low water cut-off in a pressure hot water heating system having an accumulator therein without draining down the system comprising: (a) locating the sensor in a cavity communicating with the interior of the boiler via a passageway extending beyond the sensor, said cavity being at least partially filled with water during operation of the boiler;   (b) injecting gas from a position exteriorly of the boiler into said cavity to pressurize the same to an extent so as to drive water in the cavity to a level so as to simulate a low water condition and thereby determine whether or not the sensor is operable; and   (c) releasing only a portion of the injected gas from a position spaced downwardly from the upper end of the cavity to thereby maintain a gas bubble of selected size at the uppermost end of the cavity.   
     
     
       14. In a hot water heating system having water in a boiler and a low water level sensor and safety cut-off device associated with the boiler and operable to shut off the system when the water in the boiler drops below a predetermined level, said low water sensor device comprising: (a) a housing having a cavity therein;   (b) means mounting said housing on an upper wall of the vessel;   (c) a sensor comprising an elongate electrode suspended from an insulator that projects into the upper end of said cavity and responsive to the liquid level therein, said sensor being operably associated with the heating system to shut off the same when the liquid level in the boiler drops below a predetermined level;   (d) a passageway in said housing extending downwardly from said sensor into the interior of the vessel and having an open inlet end immersed in the water during normal operation of the boiler, said passageway providing fluid flow communication between the interior of the vessel and said cavity, the inlet to such passage being at a lower vertical elevation than a lower terminal end of the sensor;   (e) valved passage means in said housing disposed exteriorly of the vessel for injecting a gas into said cavity to pressurize the cavity above the normal pressure therein; and   (f) means, located exteriorly of the vessel, for releasing the gas from said cavity, said gas release means being at a lower elevation than the insulator from which the probe is suspended.   
     
     
       15. A system as defined in claim 14 wherein said housing includes a lower portion that projects into the boiler beyond the upper wall thereof.